I used to DM in the Bahamas:
Your paperwork. Passport, driver's license, logbook and all c-cards, medical prescriptions. Fishing license, if not offered on the boat.
Cash, and plenty of it. Cash solves alot of problems. At least one credit card or debit card, so you can get more cash.
Supplimental insurance, do not dive abroad without DAN or DiveAssure insurance.
Batteries for all appliances, Walkman with earphones, earplugs, two paperback books, cell phone; several small waterproof Pelican cases to keep it all in.
Big floppy hat, sunglasses, at least 2 long-sleave t-shirts, 4 t-shirts, two sweat pants, canvas rubber-soled sneakers for shore excursions, 3 bathingsuits (2 must be dive compatible), and at least one shore-a-going outfit and sandals. Stuffable canvas bag and net bags.
Deodorant spray, athletes-foot medicine, lip-chap, plenty of suntan lotion, neosporine, small complete first-aid kit with band aids, Dramamine, babywipes, eye drops (Murine), mouthwash, gum, hard candy, chocolate bars, small travel sewing kit, skin cream, shampoo/liquid soap, twine and clothespins to hang wet clothes (you will use them), and towels. Packets of tea, sugar, salt, pepper, honey, etc..
All of your normal diving equipment, especially two each of your masks, fins (one can be full-foot for snorkeling), snorkels, and booties. Do not forget to leave home your expensive personal watch, buy a cheap, water resistant watch for daily use. Every trip someone loses a watch.
You will use everything above, for sure.